This is totally up to you. Credits are designed to help you manage the number of Fan Passes available. You may decide to purchase a limited number of credits so that only a specific number of customers can purchase your Fan Pass. Alternatively, you may wish to start off with a set number of Fan Passes available, then top up your balance in future. For example, you may choose to purchase 200 credits initially, then purchase a further 50 at a later date.
The pricing is designed to scale with higher quantities, so if you have a larger customer base, you can get a discount when purchasing more Fan Passes.